Various methods for safe connection and disconnection of high power optic amplifiers in optical links have been discussed in the prior art.
It is understood that the basic reason for shutting down lasers of the optical amplifiers is a fiber cable cut (break) or disconnection in the line; the fiber cut leads to emission of high energy light at the break point, which may cause hazardous consequences to human eyes. The most natural and widely used classic criterion indicating a fiber break is a Loss Of Signal (LOS) in the wavelengths band intended for transmitting the informational optical channels (i.e., in the C-band), being detected at the receiving point of the optical line.
In optical lines deploying power Raman amplifiers, a number of non-linear effects usually develop; therefore, the above-mentioned criterion of LOS cannot be used as a universal one for detecting fiber faults of various natures.
U.S. Pat. No. 6,373,621 to Nortel Networks corporation describes a method of operating Raman amplification pump lasers, especially in telecommunication systems, in which the Raman pump laser output powers are modulated in characteristic fashions. Parameters of these characteristic modulations may be detected at remote locations (i.e. locations along the communications fibres) even in the presence of large amounts of noise. Thus, by detecting losses of signals indicative of the characteristic modulations, breaks in the communication fibres can be detected more reliably. The disappearance of these signals can then be used to shut down the typically high power Raman pump lasers, thereby reducing the possibility of high laser powers escaping from the broken fibres. The above solution also describes an improved method of detecting the disappearance of a data signal at an amplifier unit, using a periodic filter to split the incoming signal into two streams, the first stream comprising data and noise, and the second comprising noise only. These two streams are then monitored to provide an indication of the presence or absence of the data signal alone. This form of signal detection can be combined with the technique of modulating the Raman pump laser output power to provide even further improved break detection and subsequent safe shut down of pump lasers.
An ITU-T Standard Recommendation G.664 (dated March 2003) named “Optical safety procedures and requirements for optical transport systems” proposes some examples of Automatic Power Reduction (APR) and Automatic Laser Shutdown (ALS) procedures for systems including those based upon Raman amplifiers. Appendixes 1 and 2 of the G.664 proposes using an Optical Supervisory Channel (OSC), when present in the system, to verify link connectivity for performing a restart. Since an OSC is usually operating at a safe optical power level, it can be kept “alive” on the fibre after the power has been reduced to a safe level. Restoration of OSC communication indicates full restoration of the link connectivity, after which the system can be brought back to its full operational power. In this way, it is ensured that the full operational power is only present in a fully enclosed configuration guaranteeing optical safety.
The ALS procedure concerns restart of communication in an optical link, and is based on the use of repetitive pulses to restart the system. The Standard Recommendation specifies the minimum delay between the restart pulses for various conditions (for example, between 100 s-300 s), wherein their duration (i.e., the time period when the transmitter Tx is on) can be of about (2±0.25)s.
U.S. Pat. No. 6,626,587 B1 to ECI Telecom Ltd. describes different modes of ALS procedure in optical telecommunication systems: a manual restart in a test mode, a manual restart and an automatic restart. The description is hereby incorporated by reference.
There is presently no conclusion in the prior art, which combination of criteria for shutting down and/or restarting the Raman amplifiers in the optical link could be used as the only efficient and reliable combination.